Project Hovercraft

7/31/2019 Project Hovercraft

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Project Hovercraft

The goal of this project was to build a working remote controlled hovercraft while

spending as little money as possible. I was successful, as the hovercraft works(mostly) and the only money I spent was on the batteries and propellers.

The workings of a hovercraft are fairly straightforward: one high-power motor with anairplane propeller forces air down through the hole in the center, which pushes thebottom of the hovercraft off the ground. This greatly reduces friction, allowing thehovercraft to scoot around without much trouble. It also makes it much more difficult tocontrol, however: when the hovercraft turns, it will keep traveling in a straight lineunless more thrust is applied. Since we are used to controlling cars or boats, thenewtonian behavior of a hovercraft is challenging indeed.
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The body of the hovercraft is made out of styrofoam, cut with a saw and sanded tosmooth the edges. The skirt on the bottom is made out of pieces of a plastic GAP bag,attached with Scotch tape and hot glue. The radio control unit was scavenged from anold boat I had, along with the drive motor mounted on the wooden supports, while thelifting motor was from my physics teacher. The battery is 300 mAh 6V NiMH (or NiCd, Iforget), and I have a pair of them. They were about $6-$10 each (again, forgot) andabout $12 for the 2 1/2 hour charger. The servomotor for steering was from an old RCcar I had, but I unfortunately wasn't able to find the radio unit or else I would've usedthat instead. I needed the gears in the thing so it could provide enough torque,however.Probably the single most useful thing during construction was hot glue - the entire thingis held together with the stuff and it will hold for some time to come. The physicalconstruction of the hovercraft was really the easy part... the hard part was trying to getit to work the way I wanted it to. I was plagued with numerous problems, most of themstemming from the cheap RC unit I was using.


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The first problem was with the steering. The boat had used a pair of magnets and apair of electromagnets to steer, flipping a little rudder back and forth by toggling those.The immediate problem I saw, before I even took the boat apart, was that it would onlysteer when the other (drive) channel was active. This isn't the worst of problems, but itis annoying. The problem I found with the steering method was that it used two signalsto control two electromagnets, and I wanted to run one motor in two directions instead.To do this, I had to build the interface board sitting on the front left of the hovercraft,consisting of two transistors. I had originally wanted to use an ungeared motor, so therudder would move to either side and return to the center from the force of the airbeing blown backwards. An ungeared motor didn't provide enough torque, however,and it was a bad idea to hold a motor stalled while the craft was turning. So, I took theservo from an old Street Comber RC truck I had and used that for steering. Theproblem now, however, is that it doesn't center by itself. You can see four wires comingout of the side of the servo, that are not being used. Those four wires are feedbackwires that can (very roughly) give me the position of the rudder. However, I am wayway way too lazy to design and build a more complicated board that would center it forme (that, and I determined that it wouldn't be able to center it accurately at all, so itwould be a waste).

Thus, for now, the steering works. However, because it doesn't automatically re-centerthe rudder when you let go of the stick, the hovercraft has a major tendency to spin outof control, especially considering how hard it is to pilot already.


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The next problem was with the drive motor. I want it to be able to go forwards andbackwards, since without this ability the hovercraft gets stuck fairly easily. In this

regard, I was not successful - it only goes forwards. The boat was originally run on 9V,so the way they made the motor run in two directions was by connecting one terminalto 3V and alternating the other between ground and 9V. The motor would run fast inone direction and slow in the other. Since the hovercraft runs off of 6V, this isn't reallya possiblity. I tried doing the same thing, connecting one terminal to 2.4V (the batteryis 5x1.2V cells), but it ran far too slowly in both directions. I then tried building aninterface board like I did with the steering, and it definitely should have worked. I friedstandard transistors the moment I turned it on, so something was wrong there. I figuredthere may have been too much current, so I tried power transistors. Before those friedas well, it still didn't work, so I figured something is just weird with the way the signalout works from the RC board. So, I just connected one terminal of the motor to groundand the other to the signal - when you push forward on the stick, it runs at the full 6V,and when you pull back, nothing happens.


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Skirt with hovercraft on (left) and off (right)

When I said that none of the problems had been with the physical construction, I lied.One of them was - the skirt. The skirt is the bag that fills up with air, so that the bottomof the hovercraft can deform to match whatever surface it is driving over. Now, it works

great, but it was not always thus. My first attempts at a skirt were with a flawed design- I figured I could just fold it in and tuck it under the hovercraft. This repeatedly failed,and it was not until I tried some other things that I had any luck. Turns out the secret isthat I actually had to pull the skirt back up underneath the hovercraft, to prevent it frombillowing out to the sides. To this end, tape and hot glue were very useful. If you wereto look at a cross section of the skirt when powered, it would look like a J (whereas myfirst attempts resembled more of a backwards L). Having driven it outside, it has gottena little banged up and is no longer working as well as it once did. However, thehovercraft can still drive over a variety of surfaces and go over obstacles around 1.5cm in height. I have yet to try it on water, but I hear that the friction slows it downsignificantly.

With the skirt problem resolved, the drive problem half resolved, and the steering stillquite impossible, the hovercraft is done. I'm satisfied with the result, it tought me a lotand stuff, and it's cool (the latter being the main criteria for anything I build). Maybesomeday I'll build a bigger one - I have a 1 HP cordless drill motor sitting around that'sust begging to be put to good use. We shall see.


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Closeup of the front circuitry. The entire RC board is coated in wax (since it was from aboat), and that saved me from frying it a few times... lucky me. You can also see thecapacitors I added to the lift motor for noise reduction. Before I added those, wheneverthe lift motor was turned on it would make a bunch of RF noise that screwed up thereceiver and the drive motor wouldn't stop. Took a while to figure that one out.


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The transmitter, a battery and the charger.

Project Hovercraft

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